Estimating and Reporting Grade at Bendigo
Key Points
- Most of the gold in Bendigo occurs as visible pieces of gold, greater than 1 mm in diameter (coarse gold), and is distributed very erratically within quartz reefs. This natural phenomenon is termed the nugget effect.
- The nugget effect means that no small sample, for example a single drill intersection, can accurately represent the grade of the entire reef, as small samples will tend to miss the coarse pieces of gold. To obtain a representative grade of the reef requires many small samples, perhaps 100,000 drill intersections, or several big samples, say one to two 10,000 tonne parcels.
- The challenge at Bendigo is to utilise information available from drilling in determining grade; as drilling usually precedes development, it is a highly efficient sample collection method and often provides the only source of grade information.
- The Company now uses a more fundamental approach which is better suited and appropriate to local conditions. Company geologists now use their accumulated knowledge to make assessments of size and quality of mineralisation based on the visual examination of core.
- This geological assessment is backed by assays and the knowledge gained from processing over 140,000 t of ore and a reassessment of core drilled through those mined areas.
- The Company will no longer report statistically-factored gold grades (mathematically adjusted assays). This is a very different approach and will be refined as more information is gained.
- The geological grade range estimate is based on a ranking of the key geological textures and minerals visible in the drill core: including quartz percent, quartz textures, presence of free gold and sulphides. The use of this ranking, in combination with assays, provides a meaningful method of estimating the likely gold grade range of the intercept.
